The present invention relates to ultrasound imaging with sub-arrays. In particular, combining data in different sub-arrays is provided.
Ultrasound imaging systems typically have a limited number of transmit and receive beamformer channels, such as 128, 192 or 256 channels. Multi-dimensional transducer arrays have hundreds or thousands of transducer elements, creating a mismatch of the number of elements to the number of transducer channels. To match the transducer to the beamformers, sub-arrays are defined on the transducer array. The physical size of the array or number of elements and the number of channels determines the number of elements included within each sub-array. A larger number of elements included in the sub-array requires a fewer number of cables and beamformer channels.
To make use of the signals from each of the elements within a sub-array, the signals are combined. For example, partial beamforming is applied to form a single output for each sub-array. Relative delays are applied to the signals from the elements within the sub-array as a function of the steering direction and/or focal depth. The relatively delayed signals are then summed to form a common output. The process is referred to as partial beamforming since only a portion of the array is beamformed for output to the imaging system. Final beamforming is across the array as a function of the different partially beamformed sub-array signals. The amount of relative delay applied to different elements within a sub aperture is determined by the differential in time of flight to a focal location from each of the elements within the sub-array. A greater range of time of flight, leads to a greater difference in delay. A greater difference, such as due to a larger sub-array size, may introduce errors in application of the delays, increased cost or increased complexity in the circuitry used to implement the partial beamformation.
In another approach for reducing the number of signals from an array to match the number of system channels, U.S. Pat. No. 6,676,602, the disclosure of which is incorporated herein by reference, switchably connects different elements across the entire array associated with a same delay. For different steering directions, different groups of elements are associated with a same delay. The shape of the element groups changes based on the steering without any predefined sub-arrays. The signals from elements associated with a same delay are summed together, such as using a summing node or mere connection of conductors and provided to individual channels of the beamformers.